Melt the butter in a saucepan, and brown the onion and chicken pieces in it. Add the flour and salt. Mix. Add the chicken broth and cook on a low heat for 40 minutes.

In a bowl, beat the cream and egg yolks together, and add some of the broth. Mix well. Pour this mixture into the saucepan. Stir with a spoon and serve at once, before the liquid comes to a boil.